Arctic oil and gas development

ABSTRACT

In performing oil and gas exploration and production operations in arctic regions in association with submerged well sites, a shallow draft barge equipped with appropriate mechanisms and devices is used. During periods when no ice is present over the submerged well site, the barge is moored over the well site. During periods in which ice sheets overlie the submerged well site, the barge is buoyantly supported in a pool of water which communicates through the ice sheet. As the ice sheet moves laterally relative to the submerged well site, the position of the pool is moved in the ice sheet so that the location of the barge is maintained over the location of the submerged well site within limits appropriate to the nature of the operations being performed at the submerged well site. Movement of the water pool within the moving ice sheet is accomplished by melting the boundaries of the water pool lying between the barge and the direction from which the ice sheet moves toward the barge at a rate equal to the rate of movement of the ice sheet relative to the submerged well site. The barge is movable across the top of the ice sheet, or across water or land, to and from well site locations on a cushion of low pressure air. The barge may also be used as a land based site of oil or gas well drilling or production operations.

United States Patent 1 Anders 4 1 July 31,1973

[ 1 ARCTIC OIL AND GAS DEVELOPMENT [75] Inventor: Edward O. Anders, Houston, Tex. [73] Assignee: Global Marine Inc., Los Angeles,

' Calif.

[22] Filed: Apr. 1, 1971 [21] Appl. No.: 130,092

[52] U.S. Cl 166/.5, 175/5, 114/.5 D,

114/40, 61/36 A [58] Field of Search E21b/7/12; l14/.5 D, 114/40, 41, 42; 175/5, 7; 166/.5; 61/46, 36 A [56] References Cited UNITED STATES PATENTS 3,669,052 6/1972 Schritzinger 114/42 3,670,681 6/1972 Upchurch 114/40 3,670,813 6/1972 Duncan 166/.5 3,678,873 7/1972 Bennett 114/42 3,681,927 7/1972 Duc et a1. 61/72.1

355,214 12/1886 Romaine.... 114/40 3,563,041 2/1971 Michel 61/46 3,572,273 8/1969 Wood 114/40 3,664,437 5/1972 McCulloch 175/7 FOREIGN PATENTS OR APPLICATIONS 21,844 7/1914 Great Britain 114/40 56,416 4/1936 Norway 114/40 Primary Examiner-Marvin A. Champion Assistant ExaminerRicha rd E. Favreau Attorney-Christie, Parker & Hale [5 7 ABSTRACT In performing oil and gas exploration and production operations in arctic regions in association with submerged well sites, a shallow draft barge equipped with appropriate mechanisms and devices is used; During periods when no ice is present over the submerged well site, the barge is moored over the well site. During periods in which ice sheets overlie the submerged well site, the barge is buoyantly supported in a pool of water which communicates through the ice sheet. As the ice sheet moves laterally relative to the submerged well site, the position of the pool is moved in the ice sheet so that the location of the barge is maintained over the location of the submerged well site within limits appropriate to the nature of the operations being performed at the submerged well site. Movement of the water pool within the moving ice sheet is accomplished by melting the boundaries of the water pool lying between the barge and the direction from which the ice sheet moves toward the barge at a rate equal to the rate of movement of the ice sheet relative to the submerged well site. The barge is movable across the top of the ice sheet, or across water or land, to and from well site 10- cations on a cushion of low pressure air. The barge may also be used as a land based site of oil or gas well drilling or production operations.

87 Claims, 21 Drawing Figures Patented July 31, 1973 9 Sheets-Sheet 1 HTTORNEYS i Patenled July 31, 1973 3,749,162

9 Sheets-Sheet Patented July 31, 1973 I 3,749,162

9 Sheets-Sheet 4.

Patented July 31, 1973 9 Sheets-Sheet S Patented July 31, 1973 3,749,162

9 Sheets-Sheet Patented July 31, 1973 9 Sheets-Sheet 8 I I I I I I I GL VCOL ARCTIC OIL AND GAS DEVELOPMENT FIELD OF THE INVENTION This invention pertains to method and apparatus for forming and operating oil and gas wells in arctic locations. More particularly, one aspect of the invention pertains to maintaining the position of a floating barge in a pool of water located in a movable ice sheet by the application of heat to the ice.

BACKGROUND OF THE INVENTION Description of the Prior Art Substantial known reserves of oil and gas exist in arctic regions, primarily in northern Alaska and northern Canada. Because of the extreme environmental conditions which exist in these areas, only those reserves which lie below the surface of the land, rather than below the surface of the arctic oceans, or other water bodies, are presently economically exploitable. Substantial quantities of oil and gas are known to lie in geological formations offshore from the northern shore line of Alaska and Canada, for example, but it is not practically feasible or economical to use conventional offshore exploration and production methods and equipment in developing these offshore reserves.

Several existing techniques have been successfully used in tapping offshore oil and gas reserves in temperate and tropical zones. In situations where the water depth offshore over the reserve is relatively small, say no greater than 300 ft., it is commonplace to drill the oil or gas well, and then to subsequently produce oil or gas from the completed well, by the use of a tower connected to the ocean floor and extending to above the mean water surface where the drilling or production equipment is located. Installations of this type are known as Texas Towers and are used in the Gulf of Mexico, off the western coast of the United States, and in Cook Inlet in Alaska, among other areas. The tower may be erected in place, or it may be fabricated elsewhere as a unit, towed into place, and sunk into position; these latter types of towers are hybrid semisubmersible units. It is also known to use jack-up floating platforms as towers.

In instances where the water depth exceeds about 300 ft. but is less than about 600 ft., semi-submersible platforms which resemble the bottom-footed Texas Tower, but which do not actually rest upon the ocean floor, are used. A semi-submersible drilling platform is essentially a floating stable platform which is anchored in position over the location of the submerged well site and from which the oil and gas wells are drilled. In these water depths, however, semi-submersible platforms are not practically useful to produce oil or gas from a completed well. Instead, submarine pipe lines are used to transfer oil or gas flowing from the completed well to a remote production facility located either on dry land or on a tower of the type used in water depths less than 300 ft., for example. I

It is also known to use floating vessels, or barges, of essentially conventional configuration to drill oil and gas wells in water depths in which semi-submersible platforms are used, as well as in water depths in excess of 600 ft. Where the water depth is sufficiently shallow that mooring systems may be used, a floating drilling vessel is anchored in position over the well site. In extremely deep water, however, such as water depths in excess of about 1,400 ft., the vessel may be freefloating over the well site and be maintained in position over the submerged well site by techniques referred to as cynamic positioning tehcniques.

The hazards of year-round use of offshore towers erected on the ocean floor, of semi-submersible moored drilling platforms, and of moored drilling vessels in the Arctic are substantial due to the ice problem which exists there during much of the year. A permanent polar ice pack exists over much of the Arctic Ocean and varies in extent depending upon the time of the year. During the arctic summer, the diameter of the arctic ice pack is at its smallest, with the result that areas of open water may exist between the ice pack and the northern shores of Russia, Norway, Greenland, Canada and Alaska. During the winter, however, the diameter of the ice pack expands to positions much closer to, and in some cases in direct contact with the shore line. In some areas, however, the permanent ice pack does not enlarge during the winter into direct contact with the shore line, and in these areas the water is covered by what is known as land-fast ice. Land-fast ice is an ice sheet which, because of the geography, may extend up to 25 miles offshore but is fast to or fixed to the land. In comparison, the permanent ice pack slowly rotates and circulates in the Arctic Ocean. Land fast ice, however, is not perfectly stationary throughout the entire period in which it exists in the Arctic. A land-fast ice sheet may be up to 10 ft. thick, or more, depending upon the geography involved and the time of year. Land-fast ice motion is the result of internal expansion or contraction within the ice sheet. [ts motion therefore is in random directions at random rates up to 5 ft. or so per day in response to tides, currents, and temperature changes. Studies made to date indicate that the force exerted by moving land-fast ice on a structure extending through the ice sheet from a connection to the ocean floor may vary in value from 400 lbs. to 1,000 lbs. per square inch.

Those familiar with the practices prevalent in establishing and operating offshore oil wells in warmer climates will readily appreciate that these techniques and structures are not useful in arctic locations except during relatively short periods where no ice is present over the submerged well site. It is uneconomical to use conventional offshore drilling equipment in arctic locations because of the short ice-free season available, the high cost of the equipment, and the cost and time involved in moving the equipment into and out of the arctic location. That is, the ice-free season is only about 2 months long, and it is during this season that the semisubmersible unit must be towed into place, moored, the well drilled, and the unit towed out of the area. This means that in arctic waters only one well per year, at most, can be drilled. Since such units are usually leased on a daily basis and would be idle a substantial portion of the year, the effective day rate of such units, if used in arctic operations, would be prohibitively costly. Also, semi-submersible equipment cannot practically be used in arctic environments during ice-free periods because of the considerable size of these structures which must be fabricated intact in a shipyard or the like. Shipyards capable of constructing semisubmersible drilling structures do not exist proximate to the arctic oil and gas reserves under consideration. Also, because of their extreme size, semi-submersible structures cannot be towed within reasonable periods into and out of ice-free arctic areas.

Drilling platforms erected on the ocean floor are not a practical solution to the problem of offshore oil and gas production in arctic areas because, to be economical, such structures would have to be of sufiiciently light weight that they could not withstand the lateral forces applied to them by ice during substantial periods of the year. A texas Tower of strength sufficient to withstand ice loads of the levels encountered off the northern shore of Alaska or Canada is prohibitively expensive. Thus, the use of platforms erected on the ocean floor is not regarded as a useful expedient in arctic areas because they are useful only during a short period of time during which no ice conditions exist, and they would be destroyed by ice movement during winter periods. If they were used at all, such towers would have to be erected and used only on a seasonal basis and rebuilt from year to year.

The towers and platforms used in' Cook Inlet near Anchorage, Alaska, are'of extremely heavy construction and may be used because winter ice conditions there are less severe than in arctic regions; Cook Inlet lies in the North Temperate Zone. Also, ice movement in Cook Inlet is essentially tidal and thus is of known direction so that surface piercing, bottom-footed structures can usefully be built to resemble bridge piers. Further, Cook Inlet is sufficiently close to the shipyards of Puget Sound that semi-submersible platforms, even those of very heavy construction, can be towed economically to Cook Inlet and there sunk to the sea floor to function as a tower.

Self-propelled drilling vessels can be used in arctic areas only during the ice-free season but the economics of this solution are presently undesirable because of the high daily cost of such vessels and the short length of the ice-free season.

Because the direction in which the landfast ice may move from time to time is random, it is not practical to construct bottom-footed towers capable of resisting such overturning moments applied from any direction. It is for this reason that the solutions found to be acceptable for Cook Inlet, for example, are not satisfactory in or adjacent to the Arctic Ocean, for example. Free motion of the polar ice pack is substantially greater than the strain motion associated with the landfast ice sheet.

The problems presented by forces applied by moving land-fast ice sheets to a bottom-footed structure are also pertinent to moored semi-submersible platforms and moored drilling vessels. The mooring systems used with these structures cannot withstand the loads imposed by land-fast ice sheets.

Another factor which must be dealt with in establish,- ing any procedures or structures for exploitation of oil and gas reserves in arctic regions is the unusual ecological considerations which are pertinent. The tundra, which exists at or adjacent to the areas of concern to the oil industry, is a delicate thing. The vegetation which exists inthe tundra is shallow-rooted, delicate and slow growing. This vegetation cover, however, is vital to the survival of the Arctic terrain because of the existence of permafrost below the vegetation layer. During the arctic summer, the ground thaws, but does not thoroughly dry out, over a short distance below the surface; this zone is termed the active layer. Below the thawed area lies a thick layer of permanently frozen soil known as permafrost. This frozen soil contains substantial frozen water. Since it is known that frozen water has a greater volume than water in its liquid state, it is apparent that thawing of the permafrost results in subsidence of the terrain. Subsidence of the terrain, due to destroyed permafrost, has a devastating effect upon the terrain because of the new water channels and water courses which result. It is known that the tanks and other vehicles moved over the tundra during the arctic summer in World War II destroyed the vegetation over the permafrost, and the tracks of these vehicles are now visible as the trenches having widths up to 40 ft. and depths up to 8 ft. The vegetation which grows over the permafrost'does not reestablish itself sufficiently rapidly to heal the wounds made in the tundra by man.

For these reasons, the governmental authorities having cognizance over arctic regions carefully regulate the manner in which vehicles and equipment are moved across the tundra. These regulations have the practical effect of limiting the operations of those dcsiring to develop and exploit oil and gas reserves in the Arctic. Specifically, permits are now required from the governmental authorities to traverse the trundra with a vehicle. If permits are issued, the permit may restrict the route which the vehicle is to follow, or limit the number of times the vehicle may use the same route in traversing the tundra. Thus, vehicles which have very low effective footprint pressures (i.e., load per unit area) upon the tundra are far more desirable for arctic use than vehicles which have high effective footprint pressures. Vehicles having a low footprint pressure will be permitted to travel over the same route many more times than vehicles having a high footprint pressure.

It is apparent from the foregoing, therefore, that a substantial need exists for the development of procedures and equipment capable of use in the Arctic on a year-round basis for the location, development and production of oil and gas from submerged'areas. Because the long-term structural properties of ice are not known or understood, it is desirable that the equipment used in association with offshore arctic wells not rely upon the ice sheet for support. For maximum economical benefit, it is desirable that any oil-and gas drilling and production equipment suitable for arctic use be movable from place to place relatively easily, even across land. Therefore, it is desirable that the structure, during movement across land, develop a very low effective footprint pressure on the tundra w as to eliminate the danger of altering ecology due to environmental damage to the tundra active layer in the thaw period, and thus qualify for transit permits issued by the cognizant governmental authorities.

SUMMARY OF THE INVENTION This invention provides procedure and equipment meeting all of the desired criteria outlined above and useful on a year-round basis in establishing and operating submerged oil and gas wells in arctic environments, even in areas covered by land-fast ice and the like. The structures contemplated by this invention are simple, effective, protective of the environment and, most importantly, economical.

In terms of method, and stated broadly, one aspect of this invention pertains to a method for performing operations at and in association with a submerged marine location overlaid by a sheet of ice susceptible of movement relative to the submerged location. The method includes the step of providing a buoyant platform from which desired operations may be performed. The platform is buoyantly floated in a pool of water which communicates through the ice sheet over the submerged location. Desired operations at and in association with the submerged marine location are performed from the platform while the same is in a buoyantly floating state. The position of the platform over the submerged location is maintained during movement of the ice sheet relative to the submerged location within limits appropriate to the operations performed at and in association with the submerged location. Because the platform is buoyantly supported in a pool of water which communicates through the ice sheet, the weight of the platform is supported buoyantly and no portion of the weight of the platform is supported for any extended period by the ice sheet itself. Preferably, the position of the pool in which the barge floats is maintained over the submerged location by heating the water in the pool, at least adjacent those boundaries of the pool toward which ice may move, to a temperature sufficient to cause the pool to move in a direction opposite to and at a rate equal to the movement of the ice sheet. In effect, as the ice sheet moves over the submerged marine location, the pool is moved in the ice sheet so as to stay stationary over the submerged location. Preferably, the platform is moved into the desired position over the submerged location, particularly during periods in which ice is present, by supporting the platform on a cushion of air and towing the platform across the ice by suitable tractors or the like.

Stated even more generally, the present invention provides a method for maintaining a floating structure in a substantially fixed position in a pool formed within ice subject to lateral motion. The method includes the step of melting a lateral boundary of the pool on a side thereof opposite the direction of ice motion; this is done for maintaining the pool, and the structure which floats in the pool, in a substantially fixed position irrespective of ice motion.

This invention also pertains to apparatus for performing these procedures. The structural aspects of the invention are set forth in the following description.

DESCRIPTION OF THE DRAWINGS The above mentioned and other aspects of the invention are set forth more fully in the following description of the presently preferred embodiments of the invention, which description is presented with reference to the accompanying drawing wherein:

FIG. 1 is a perspective view of a drilling platform according to this invention on location in an ice sheet over a desired submerged location;

FIG. 2 is an elevation view, partially in cross-section, of a drilling platform according to the invention;

FIG. 3 is an enlarged fragmentary perspective view, partially in cross-section, of a portion of the drilling platform shown in FIG. 1;

FIG. 4 is an elevation view illustrating the movement of a platform according to this invention on an air cushion out of a supporting well formed in a thick ice sheet;

FIG. 5 is a schematic diagram illustrating a closedloop heating and refrigerating energy transfer system for a drilling or production platform according to this invention;

FIG. 6 is a schematic illustration of an open-loop energy transfer system for a drilling or production platform according to this invention;

FIG. 7 is a fragmentary cross-sectional elevation view of a portion of a platform according to this invention and illustrates how open and closed loop energy transfer systems may be used in combination with each other;

FIG. 8 is a cross-sectional elevation view of another platform according to this invention and particularly illustrates the manner in which a connection is maintained through an ice sheet from the pool in which the platform is buoyantly supported;

FIG. 9 is a simplified top plan view of the mooring in an ice sheet of a platform according to this invention;

FIG. 10 is a fragmentary elevation view, partially in cross-section, of the air cushion mode of operation of a platform according to this invention;

FIG. 11 is an enlarged fragmentary elevation view of a portion of FIG. 10;

FIG. 12 is an enlarged fragmentary cross-sectional elevation view showing the closure of a central well through a drilling platform for the purposes of equipping the platform for movement of an air cushion;

FIG. 13 is an elevation view of a platform according to this invention equipped for the production of oil from a submerged well disposed below an ice sheet;

FIGS. 14 through 17 are cross-sectional elevation drawings illustrating, in sequence, a method of positioning of a platform according to this invention in a DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Introduction As will be apparent from the following more detailed description, an operations platform has a buoyant barge-like hull. The hull is arranged to float with shallow draft. The platform carries such equipment as necessary to enable performance of desired operations (exploration, drilling, or production, for example) at or in association with an oil or gas well site.

Where the platform is used in the winter over a submerged well site, the platform is floated in a pool formed in an ice sheet over the site. The pool has communication through the ice sheet so that the weight of the platform is entirely buoyantly supported in use; the ice sheet is not relied upon for support of the platform. The platform hull includes energy transfer arrangements for transferring chemical, thermal and mechanical energy from within the hull to the water pool and the surrounding ice sheet. Preferably the transferred energy is predominantly thermal energy, but in some situations it may be appropriate to use only or predominantly mechanical or chemical energy or a combination thereof. Transferred energy is used to maintain the pool in an unfrozen state and to melt or errode away the pool walls to compensate for movement of the ice sheet relative to the submerged well site. The platform is moored to the ice sheet, and not through the ice sheet to the ocean floor. The position of the platform in the pool is maintained as desired by paying out or taking in the mooring lines. Also, the mooring lines are payed out or taken in as necessary, in conjunction with the ice melting operation, to effectively move the pool in the ice sheet so that the pool and the platform stay stationary over the well site irrespective of movement of the ice sheet.

An air cushion skirt is removably connected around the circumference of the bull to enable air cushion movement (i.e., ground effect operation) of the platform across ice, land or water when pressurized air is supplied to the space below the hull within the skirt. The skirt is removed during use of the platform in a pool formed in an ice sheet.

To facilitate use of the platform as a land-based unit, it is preferred that the energy transfer arrangements associated with the platform bottom be reversible for cooling the hull bottom to prevent thawing of permafrost below the hull when the hull rests on tundra.

General Arrangement Referring initially to FIGS. 1 and 2, an air cushion thermal drilling platform is positioned in a well (i.e., recess) 11 formed in a thick ice sheet 12 which extends over the surface of an ocean 13 or the body of water in an arctic area. As used herein, the term thick ice refers to ice approximately 2 ft. or more in thickness, whereas thin ice" refers to ice 2 ft. or less in thickness. These figures, however, are relative, and the terms thick or thin really refer to situations where the ice sheet cannot or can be penetrated immediately, or in relatively short order, by the weight of the air cushion platform disposed on the ice sheet. The significance of the terms thick or thin as applied to the ice sheet will be more clearly apparent from the following description pertinent to FIGS. 14 through 18, for example.

As shown in FIG. 2, drilling platform 10 is positioned over a desired submerged location on the floor 14 of the ocean at which an oil well 15 is being drilled pursuant to operations carried out from the drilling platform.

As illustrated in FIG. 4, for example, an air cushion drilling platform 10 includes a rectangular barge 17 having a hull to which a removable air cushion skirt 84 (described in greater detail hereinafter) is detachably connected around the periphery of the hull. When the air cushion skirt is connected to the hull of platform 10 and air is supplied via the skirt to the chamber formed beneath the barge within the confines of the skirt at low pressure, the barge is supported on a cushion of air with its lower surface 19 approximately 6 ft. above the water surface (or above the surface of the ice or land) so that the platform may be moved from place to place over the water, ice, or land. To implement the air cushion mode of movement of the drilling platform from one drilling site to another, air is supplied at superatmospheric pressure to within the skirt from a plurality of fans or blowers (see, for example, blower 21 shown in FIG. 10) which are driven by suitable prime movers incorporated within platform 10. As used herein the term prime mover encompasses a gas turbine or an internal combustion engine which may be either an ottocycle engine or a diesel-cycle engine.

As is apparent from FIGS. 1 and 2, the hull of barge l7 constitutes a major structural element of drilling platform 10 and is. of substantial width relative to its length in furtherance of achieving a barge which floats with shallow draft. The shallow draft feature of the barge is desired because of several considerations which include large clearance during air cushion mode operation of the platform, low footprint pressure of the platform during air cushion mode operation, low thermal requirements during buoyant mode operation in an ice sheet, and stability during both air cushion and buoyant modes of operation. It' is preferred that the platform have substantial planform (horizontal) area so that the amount of air pressure necessary to raise the barge hull a desired distance above the ice surface during air cushion mode operation, for example, be on the order of 150 to 225 lbs. per square foot gauge pressure. As an illustrative example, a presently preferred em-' bodiment of this invention involves a barge structure having a length of 240 it, a beam of ft. with an air cushion pressure of l50 lbs. per square foot; the keel of this barge structure has a clearance of about 6 ft. above the ice or land surface during air cushion mode operation.

The large width of the barge 17 relative to its length is also desirable to assure adequate stability of the drilling platform in its buoyant and air cushion states and to equalize the heat transfer requirements of the barge during ice melting operations, particularly lateral ice melting operations, as will become apparent from the following description. It is sufiicient at this point, however, to state that if the barge were square, i.e. had a length-to-beam ratio of approximately 1, the heat demand characteristic of the platform would be the same in all lateral directions. On the other hand, it is desirable that barge 17 be capable of being towed in a fully buoyant state during ice-free conditions, and for this reason a structure having a length-to-beam ratio greater than l is desired. It is apparent, therefor, that the actual length-to-beam ratio of any barge according to this invention is actually a compromise between several competing design factors.

FIG. 1 illustrates that the deck 23 of barge 17 is enclosed within a protective insulated deck house enclosure 22 which covers essentially the entire area of the deck. The deck house has been deleted from most of the accompanying figures for purposes of clarity and simplicity of illustration. It is to be understood, however, that the deck house is a significant feature of the drilling platform and is virtually necessary to enable this invention to be practiced in arctic environments on a year-round basis. Within the deck house, on and within the structure of the barge, is located all the equipment normally encountered in a conventional floating drilling vessel. Such equipment includes a derrick tower 25 which extends upwardly from the barge deck over a well 26 (FIG. 2) formed through the center of the barge. Well 26 is located essentially amidships in order that the drilling platform may be stable during air cushion -mode movement of the platform. Derrice tower 25 is hinged adjacent its base, as at 27, above the roof of deck house 22. Accordingly, the derrick tower is hingable into a horizontal stowed position as illustrated in phantom lines in FIG. 2 above the roof of the deck house. The derrick tower is a component of a drilling facility of conventional configuration and outfitting which includes a supporting foundation 28 erected on the main deck of barge 17 over center well 26 to place a drilling rig floor 29 at a location elevated above barge deck 23. A conventional rotary table 30 is included in the drilling rig floor in a position centered over center well 26.

As shown in FIGS. 1, 2 and 9, a mooring winch 32 is located adjacent each'corner of barge 17 on the main deck of the barge, preferably within deck house 22. Additional equipment includes suitable storage tanks 33 within the deck house for drilling mud, cement and other materials normally required during offshore drilling operations, a pylon-type cargo handling crane 34 located outside the deck house, and suitable racks 35 for storing drill pipe and oil well casings. Doors 38 in the deck house roof provide access for the crane 34.

Not shown, but also included within the deck house or,

within the hull of barge 17 may be quarters for the crew of the barge and the other personnel necessary to operation of a conventional offshore drilling vessel, and other cargo spaces and machinery spaces as appropriate.

FIGS. 1, 2 and 3 show that it is preferred that the general configuration of barge 17 includes upwardly and outwardly sloping sides and ends, and a flat bottom. The sloping walls of barge l7 facilitate the design of suitable removable air cushion skirts for the barge and also reduces the resistance of the barge when being towed in a conventional manner.

Ice Sheet Operation (General) As noted above, there is presently very little infonnation available concerning the long-term structural properties of ice. All that is known is that ice, when subjected to load, has visco-elastic properties in that it behaves somewhat like a viscous medium and somewhat like an elastic medium. The extent to which ice under load behaves as a viscous medium, or an elastic medium, is dependent upon several factors, including the temperature of the ice, the temperature gradient across the ice, the extent to which impurities are present in the ice, and the crystal structure of the ice, the crystal structure of the ice being a function of the manner in which the ice was formed initially. The net result is that reliance upon a sheet of ice as a structural foundation is an inherently treacherous and risky operation. For these reasons it is not advisable to erect a drilling installation on an ice sheet over a submerged location and to drill an oil well or gas well through the ice sheet from such an installation. The uncertainties and hazards of the use of ice as a foundation for a drilling rig underlie the disadvantages of the prior thinking which tended to focus attention upon conventional offshore drilling structures for use in exploiting arctic oil and gas reserves. As previously explained, the conventional offshore drilling techniques and structures are impractical and uneconomical in arctic situations because of the extremely adverse environmental conditions prevalent.

An inspection of FIG. 2 will show that during use, no portion of the weight of drilling platform 10 is carried by ice sheet 12. Instead, the total weight of drilling platform 10 is buoyantly supported by a pool of water 36 which fills well 11 formed in the top of the ice sheet. Pool of water 36 communicates to ocean 13 below the ice sheet through a hole 37 formed in the ice sheet below barge center well 26. Therefore, as the platform sets down into pool 36 from its air cushion to its buoyantly supported state, a minor portion of the volume of water present in well 11 may be displaced out over the top of the ice sheet, but by far the majority of the volume of such displaced water is transferred back into the ocean through hole 37. The net result is that the h ll portion of the ice sheet between the bottom pool 36 and the ocean 13 is freely floating and supports no portion of the weight of the drilling platform. Thus, the ice sheet is not called upon during use of the drilling platform (after conditions are stabilized following arrival of the platform at the drilling site) to carry any portion of the load of the platform. All that is necessary, therefore, is to maintain a quantity of water in pool 36 sufficient to allow the barge to float clear of the bottom of well 11 and to maintain a communication between pool 36 and ocean 13 through the ice sheet.

The pool of water around the barge in ice well 11 is maintained by the application of energy to the ice sheet from the platform through the hull in sufficient quantities to erode or melt the ice faces moving toward the hull at appropriate rates. As noted above, the applied or transferred energy may be in the form of thermal, mechanical or chemical energy, or a combination of these energy forms. Energy transfer is also provided to prevent the water in the ice well from freezing. Vessel position is also maintained by using suitable operating procedures to maintain hole 37 through the ice during drilling operations. It is also desirable that hole 37 be maintained because drilling operations are carried out from the vessel to well 15 through this hole, as shown in FIG. 2. To implement drilling operations, hole 37 is maintained of sufiicient diameter below barge center well 26 to permit a suitable conventional landing base 39 to be conveyed from the drilling platform to the ocean floor during the initial stages of formation of well 15. Hole 37 is also maintained to allow a blow-out preventer 40 and other conventional equipment to be moved into position on landing base 39 via suitable guide wires 41 and load transfer wires 42 extended between the landing base and the barge, and between the blow-out preventer and the barge, respectively, in a conventional manner. Hole 37 through ice sheet 12 is also required since communication between the drilling barge and the well via a riser pipe 43 is required, in a conventional manner, to permit flow of drilling mud between the well and the drilling rig carried by barge 17.

In FIG. 2, a mechanism for heating the exterior surfaces of the barge within ice well 11 to maintain water pool 36 is represented schematically by elements 44 which signify heating coils disposed in contact with the inner surfaces of the barge keel and side walls and through which hot water, or some other heating medium, may be circulated.

A plurality of prime movers, not shown in detail, are incorporated within the facility provided within deck house 22. These prime movers are provided for operation of the air blowers necessary for air cushion mode operation of the platform, and for powering the rotary table and other equipment present in the drilling rig. In a 240 ft. long by ft. wide drilling barge according to this invention, the prime movers may be four caterpillar D-399 diesel engines, for example, a 725 hp die sel or four Solar Saturn gas turbines, for example, or a combination of diesel engines and gas turbines. The waste heat present in the exhaust from these engines and turbines is sufi'icient to provide ample heat for maintaining pool of water 36 in a liquid state and also for heating the submerged side surfaces of the barge to compensate for movement of the ice sheet laterally relative to well 15. Heating of the ice sheet around the periphery of well 11 for the purposes of melting the ice,

rather than merely maintaining water pool 36 in a liquid state, is another aspect of this invention which is addressed to the probiem of lateral movement of the ice over a submerged well site, for example.

It has been ascertained through tests that land-fast ice in the McKenzie River Delta region during the winter months, for example, may move laterally in random directions at rates up to ft. per day. It has been ascer tained through experience with floating drilling vessels that the tolerable lateral movement of the drilling vessel relative to a submerged well site, during drilling operations in which a riser pipe 43 is connected between the vessel and the submergeed well site, is about percent of the-water depth. This limit on lateral movement of the drilling vessel is determined by the elastic properties of the riser pipe. Deflection of the riser pipe, by lateral movement of the vessel through a distance greater than about 10 percent of the water depth, has been found to produce bends, kinks and other damage in the riser pipe and also in the mechanisms connecting the riser pipe to the landing base and the well. The water depth in many of the land-fast ice areas adjacent to but offshore from the mouth of the McKenzie River, for example, is less than 50 ft., in which region lateral movement of the ice may be as much as 5 ft. per day. It should be understood that mention of movement of the ice sheet at a rate of 5 ft. per day does not necessarily mean that the ice will move 5 ft. in any given direction before reversing its direction of movement. Quite to the contrary, it has been found that, in some instances, a given location on a land-fast ice sheet may move at a rate of 5 ft. per day for more than 24 hours, although admittedly such extended movements of the ice sheet at these rates is not the usual situation.

In light of the immediately preceding remarks, therefore, this invention comprehends, in effect, the movement of water pool 36 across ice sheet 12 in a direction opposite to and at a rate equal to the movement of the ice sheet relative to landing base 39, for example. The effective movement of the water pool across the ice sheet is accomplished, in a preferred practice of this invention, by melting and/or eroding away those boundary walls of water pool 36 adjacent to the direction from which the ice moves toward drilling platform 10 at a rate equal to the rate of movement of the ice sheet toward the drilling platform. As a result, water pool 36 effectively stays stationary over landing base 39 throughout movement of the ice sheet in random directions relative to the landing base.

Energy Transfer Systems FIG. 5 illustrates, in schematic form, a closed-loop heating and refrigerating system 45 which is useful in air cushion drillijng platform 10 or in any platform, whatever its purpose, according to this invention. As a heating mechanism, system 45 is useful for melting the walls of the water pool 36 to enable the water pool to stay stationary over a submerged well site during movement of the ice sheet. As a refrigerating mechanism, system 45 is useful for enabling drilling platoform 10 to be used on land in arctic regions in a manner described more fully in the following remarks.

As described above, drilling platform 10 includes a plurality of prime movers 46 which may be gas turbine engines or internal combustion engines of eithr the otto or diesel cycles. Each prime mover is equipped with an air intake duct 47 communicating through deck house 22 to the atmosphere, and an exhaust duct 48 connected between the prime mover and through the deck house to the atmosphere. A conventional heat exchanger 49 is provided in association with each exhaust duct 48 from a prime mover so that a suitable heat transfer fluid circulating through the heat exchanger may be raised in temperature by the hot gases moving through the exhaust duct in response to operation of the prime mover. Water may be used as a heat transfer medium in system 45 but it is preferred that a mixture of ethylene glycol and water, or some other suitable heat transfer medium, such as one of the Freon heat transfer fluids, may be used. System 45 also includes a plurality of heat exchange coils 50 which are disposed in intimate heat transfer contact with the inner surfaces of the submerged walls of barge 17. Heat exchange coils 50 are provided to implement the transfer of heat through the barge to and from the surroundings of the barge. The coils are connected to heat exchanger 49 via a manifold 51 and suitable throttle valves 52, and via check valves 53 to the intake of a circulating pump 54 which discharges via a valve 55 to heat exchanger 49. One or more heat exchange coils 50 are provided in each of different discrete heat transfer areas associated with the bottom and side walls of the barge.

System 45 also includes, for purposes of refrigeration, a compressor 56 and a condenser 57 which are connected in series relationship to each other in parallel with heat exchanger 49 in a closed-loop circuit which includes manifold 51, heat exchange coils 50 and pump 54. Suitable valves 58 are provided in system 45 to isolate compressor 56 and condenser 57 when the system is used in its heating mode, and to isolate heat exchanger 49 when the system is being used in its refrigeration mode. It is apparent that heat exchange coils 50 function as heating coils when heat transfer fluid is circulated through heat exchange device 49 during heating mode operation of the system. On the other hand, heat exchange coils 50 function as refrigeration coils when the heat exchange medium is circulated through the compressor and condenser during refrigeration mode operation of the system. As set forth more clearly in the succeeding description, heat exchange coils 50 are installed at least over the inner surfaces of the flat bottom of the barge l7 and may be provided, if desired, on the inner surfaces of the sloping side and end walls of the barge.

In FIG. 3 it can be seen that heat exchange coils 50 are disposed in intimate contact with the inner walls of the flat bottom of barge 17 and with the-sloping side walls of the barge. FIG. 3 also shows that those spaces within the barge which are occupied by heat exchange coils 50 are separated from the remaining interior spaces of the barge by suitable insulation material 59 to maximize the heating or refrigerating efficiency of system 45.

An open-loop heat transfer system 60 is shown in FIG. 6 and includes one or more prime movers 46 (only one prime mover is shown in FIG. 6), each equipped with an air inlet duct 47 and an exhaust duct 48 fitted with a heat exchange device 49 so that a substantial portion of the waste heat present in the exhaust from the prime mover may be recovered for use in system 60. In the case of system 60, the heat exchange fluid used is sea water which is introduced into the system through a sea chest 61 connected preferably through the flat bottom of barge 17. The inlet of a pump 62 is connected to the sea chest and discharges sea water under pressure to heat exchange device 49 where it is heated and then passed to a manifold 63. Hot sea water passes from manifold 63 along separate paths, each of which includes a throttle valve 64 and a plurality of water discharge nozzles 65 represented schematically in FIG. 6 by a single nozzle element. A throttle valve is provided for each discrete heat transfer area designated for a corresponding portion of the exterior surface of the barge hull. Discharge nozzles 65 are located in the hull of barge 17 so that water introduced to the nozzles is discharged to the exterior of the barge at rel atively high velocity into water pool 36 and against the walls of ice well 11. System 60 is a mechanical and thermal energy transfer system in that the velocity of the discharge from nozzles 65 operates to erode the walls of well 11 as a supplement to the melting produced by the heat energy in such discharge.

FIG. 8 illustrates another arrangement which may be use to provide heating of the bottom and side surfaces of barge 17 in furtherance of this invention. Barge 17 is equipped with an inner bottom tank 78 and separate wing tanks 79 along the side, front and rear ends of the barge, respectively. The outer walls of barge 17 define major portions of the boundaries of tanks 78 and 79. Adequate temperature levels for maintaining ice well 11 at the appropriate depth may be achieved merely by circulating hot water, or a mixture of water and ethylene glycol, through inner bottom tank 78. In the case of wing tanks 79, however, the greater requirements for heat transfer from the barge to the ice sheet require that the heated liquid be agitated somewhat. Accordingly, a perforated air supply pipe 80 is provided along the lowermost extent of each of wing tanks 79, and an air vent tube 81 is provided between the top of each wing tank and the exterior of the tank. Compressed air is supplied to pipes 80 to emerge from pipes and percolate upwardly through the hot liquid in the tank, and then to emerge from the tanks through vent tubes 81. The percolation of air from tubes 80 to vents 81 induces circulation of the hot liquid in the tanks to provide the desired heat transfer characteristic from the barge to the adjacent ice faces.

Where some or all of the prime movers present aboard the platform are gas turbines, the energy transfer systems shown in FIGS. 20 and 21 may be used to advantage. Energy transfer system 160, see FIG. 20, includes a gas turbine 161 having a discharge end 162 from which exhaust gases flow at high temperature and velocity. Typically, the turbine exhaust temperature is about 1,000 F. A portion of the turbine exhaust gases are collected by the belled end 163 of a duct 164 which is disposed substantially coaxially of the turbine to the rear of the turbine a distance sufficient to prevent undue interference (by way of backpressure generation) with the operation of the turbine. The other end 165 of duct 164 discharges to the intake opening 166 of blower 21. The quantity of turbine exhaust gases permitted to flow through duct 164 per unit time is regulated by a damper 167 installed in the duct. Blower 21 discharges air at about 2 psig via a duct 168, also equipped with a closable damper valve 169, to an air skirt air supply plenum 85 which is described hereinafter in greater detail.

End 165 of turbine exhaust duct 164 only partially closes the inlet opening to blower 21. Accordingly, when the blower is operated, both during air cushion operation of the platform and during floating operation of the platform, the gases discharged by the blower are at a temperature between ambient temperature (which may be as low as 65 F.) and the temperature of the turbine exhaust. The adjustment of damper 167 controls the temperature of the blower discharge.

A branch duct 171 extends from duct 168 between the blower and damper 169 to a pressurized air header 172 within barge 17. A closable damper valve 173 is provided in branch duct 171. A plurality of passages 174, each of which may be branched if desired, extend from the header to respective ones of a plurality of gaswater discharge assemblies 175 installed in the submerged portion of the walls of the barge to discharge to the exterior of the barge. Preferably, as noted above and as more fully explained below, assemblies 175 are arranged in discrete energy transfer areas of the barge hull surfaces. Only one of assemblies 175 is shown in FIG. 20. A closable damper valve 176 is provided in each passage 174.

As shown in FIG. 20, each gas-water discharge assembly 175 includes a venturi 178 defined by the terminal portion of corresponding passage 174. A water discharge nozzle 179 is disposed substantially coaxially of the venturi adjacent the entrance end of the venturi. The discharge end of the venturi is flared so as to cooperate with water discharged into the venturi by nozzle 179 at high pressure to cause hot low pressure gas to be educted via passage 174 from header 172. Nozzle 179 is defined at the end of a pipe 180 which extends to a pressurized water manifold 181 coupled to the discharge of a pump 182. The pump intake is connected to the discharge end of a suction pipe 183 which has an inlet end disposed below the light-weight waterline of barge 17 in centerwell 26. A pipe 180 is provided for each of assemblies 175 and each such pipe preferably includes a throttle valve 184.

Each of assemblies 175 operates to discharge mechanical and thermal energy through the barge hull to the adjacent face of ice well 11. Mechanical energy is provided by the velocity head of the warm gas-water mixture discharged to the ice, and such energy is usefully expended in erosion of the ice sheet. The thermal energy in the discharge mixture melts the ice sheet and keeps the water in pool 36 above the freezing point.

As an example, the discharge from blower 21, during floating operation of platform 10, may be regulated to have a temperature of, say, F.; this discharge is at 2 psig. The water supplied to pump 182 is at about 28 F. and the discharge pressure of the pump may be at 35 psig. Because of the pressure differential between manifold 181 and header 172, substantial warm gas from the header will be entrained in the water discharged from each of assemblies 175. The temperature of the gas-water mixture discharged from the hull through each of assemblies will be significantly above 28 F. The precise temperature of each gas-water discharge mixture is determined by the setting of valves 176 and 184 for each assembly 175. It is also apparent that the adjustment of valves 176 and 184 determines the amount of thermal and mechanical energy transferred per unit time from the barge to the ice sheet via each of assembles 175.

Especially where the ambient temperature is very low, it may be desirable to use chemcial energy to assist in removal of ice around pool 36 and to prevent pool 36 from freezing. As shown in FIG. 20, a reservoir 186 for ethylene glycol, for example, is provided in barge 17 and is connected by suitable conduit 187 to the pressurized water system. Thus, the discharge from each of assemblies 175 may contain a suitable portion of ethylene glycol which is effective in pool 36 to lower the freezing point of the pool water and to assist in the ice melting process.

Energy transfer system 160 is also useful during air cushion operation of the platform. As set forth more fully below, air skirt 84 preferably is fabricated in principal part of rubberized fabric which tends to become stiff and brittle, so as to tend to crack when flexed,-at low temperatures. The structures provided by this invention are intended for use in conditions of extreme cold, say 65 F. or colder. During air cushion operation of a platform equipped with an energy transfer system 160, valve 173 in duct 171 is closed and valve 169 in duct 168 is fully opened. Damper 167 in duct 164 is set so that the temperature of the air supplied to air skirt 84 is about 30-F., i.e., slightly below the freezing point of water for the pressure encountered within the skirt below the barge. This temperature of about 30 F. is desired to maintain the skirt material flexible, thereby extending skirt life under extremely adverse conditions of use, and to prevent condensation and freezing of water vapor on the skirt and the hull, thereby preventing the buildup of unnecessary weight against which the blowers must act. Water vapor is present in the exhaust from turbine 161 and tends to freeze in blower 21, duct 168 and plenum 169, but the high velocity of the relatively dry gases present in these areas causes such water vapor as may condense and freeze to sublime almost immediately back to water vapor at a temperature of 30 F. or so.

P16. 21 is a schematic diagram of another energy transfer system 190 which may be used where the prime movers of the platform include a gas turbine 161. System 190 is arranged for use only during floating operation of the platform in a well formed in ice sheet 12. An energy conversion unit 191 is mounted substantially coaxially of turbine 161 adjacent the discharge end of the turbine. The conversion unit has an open end 192 adjacent the turbine and an internal chamber 193 to which the open end of the unit communicates. The conversion unit is disposed sufficiently close to the turbine to receive a substantial portion of the turbine exhaust gases. Because the maximum horsepower requirement of drilling platform is that associated with air cushion operation rather than floating operation and use of the drilling equipment on the platform, the energy conversion unit may be placed sufficiently close to the turbine to reduce the efficiency of the turbine; this result is permissible since the turbine, in this instance, is used to provide hot high pressure gas, not primarily for the production of power.

Energy conversion unit 191 operates to convert the velocity energy of the turbine exhaust gases entering the unit to pressure energy within chamber 193. In effect, the unit serves much the same purpose a a compressor for the exhaust from the turbine.

Chamber 193 'is connected to a pressurized gas header 172 in the barge by a duct 194. The connection of the duct to unit 191 is severable, as at 195, so that the conversion unit may be decoupled from the turbine during air cushion operation of the platform when maximum turbine efficiency is required to satisfy the power requirements of all blowers 21 of the platform. Header 172 is connected via a plurality of passages 174 to a plurality of gas-water discharge assemblies which are substantially in accord with the preceding description. Thus, each assembly 175 includes a venturi 178 anda pressurized water nozzle 179. Water at high pres sure is supplied to the nozzle via a corresponding pipe 180 from a manifold 181 connected to the discharge of a pump 182 which has its suction in centerwell 26 below the barge shallow draft waterline. A throttle valve 184 is provided in each water line 180.

Discharge assemblies 175 are grouped in discrete energy transfer areas of the barges exterior surfaces.

System includes a cold air, high pressure duct 197 which has its inlet end connected to turbine 161 between the compressor and turbine sections of the turbine, as shown in FIG. 21. A thermostatically con trolled throttle valve 198 is present in duct 197 and is responsive primarily to ambient temperature to control the quantity of cold air flowing through the duct. Duct 197 has a plurality of outlets 199, preferably one in each hot gas passage from header 172 to a discharge assembly 175.

As an example, air at an ambient temperature of as low as 65 F. or so is taken into gas turbine 161 and discharged from the turbine at about ambient temperature but at about 60 psig through duct 197. The exhaust from the turbine is at high velocity and has a temperature of about l,000 F. A portion of this exhaust is converted in unit 191 to gas at the same temperature with a pressure of about 60 psig in chamber 193 and in header 172. In passages 174, however, thehot gas from the header and the cold air from duct 197 are mixed to provide a gas-air mixture having a pressure of about 60 psig and a temperature of about 160 F. This mixture is combined with water at 60 psig and about 28 F. in assemblies 175 and discharged at high velocities from the barge to ice sheet 12 around well 11. By appropriate adjustment of valves 176 and 184, the quantity of thermal and mechanical energy discharged per unit time by each of assemblies 175 can be selected as desired for most effective use of system 190.

Maintenance of -Water Pool Position FIG. 9 is a top plan view of an air cushion drilling platform 10 moored in well 11 formed in the upper surface of ice sheet 12. A mooring line 67 extends from each of winches 32 to an anchoring device 68 fixed in the ice sheet at a desired distance from the platform. To facilitate further description, respective ones of mooring lines 67 are designated as mooring lines 67a, 67b, 67c and 67d, respectively. Anchoring devices 68 may be simple wooden piles or the like frozen into ice sheet 12 at the desired distance from well 11. his pre ferred that, at least when the platform is initially positioned in well 11, anchors 68 be so positioned, relative to the four corners of the platform, that mooring lines 67 extend from the paltform along lines which make an angle of 45 to the center line of the platform. That is, the mooring lines extend in substantially orthogonal horizontal directions from the platform.

It is emphasized thatplatform 10 is moored via mooring lines 67 to the ice sheet, rathre than through the ice sheet to anchors disposed on the ocean floor. In this particular, among others, platform 10 is seen to be different from conventional floating drilling vessels or barges which are moored to the ocean floor. Platform 10 instead is moored only to the ice sheet, and for this reason lateral movement of the ice sheet imposes no load upon the platform mooring system. If mooring lines 67 were to extend through the ice sheet to anchors on the ocean floor, movement of the ice sheet would impose loads on the lines sufficient to part the lines (assuming the anchors hold) or sufficient to pull the anchors out of or along the ocean floor. Also, through-ice mooring lines would freeze into the ice so that ice movement would alter the mooring line path between the platform and the anchor from the usual catenary curve; this would make it very difficult, if not impossible, to monitor or estimate mooring line load from the platform. Mooring line load is regularly monitored or estimated to determine when line should be paid out to prevent parting of the line. Therefore, through-ice moorings in the arctic are to be avoided because they directly or indirectly result in loss of the mooring.

It is preferred that anchors 68 be established at positions which are spaced substantially from the platform when the platform is first set down in or on the ice sheet over the desired submerged location. The initial paid out length of mooring lines 670 to 67d should be substantially greater than the maximum total distance it is expected pool 11 be moved in any direction in the ice sheet over the period required to carry out the desired operations in association with the submerged location. If, however, this expected maximum total distance should be exceeded to any significant extent while the platform is in position in pool 36, the anchors are reset in the ice sheet at a greater distance from the platform.

Assume that barge 17 of platform has a front end 69, a port side 70, an aft end 71 and a starboard side 72. Assume also that the ice sheet moves relative to the submerged well site below barge center wall 26 from a direction approaching the front port quarter of the barge, i.e., along the line of mooring line 67a. In order that the position of barge center well 26 be maintained essentially stationary over th submerged well site during movement of the ice sheet from this direction (represented by the arrow in FIG. 9), it is necessary that the position of ice well 11 be moved across the surface of the ice sheet in the direction of mooring line 67a toward the anchor to which mooring line 67a is connected. It is also desired that the basic attitude of the barge in the ice sheet relative to compass directions be maintained as the barge is effectively moved across the ice sheet. Accordingly, it is desired to translate ice well I 1 across the ice sheet in a direction directly opposite to the direction of ice movement and at a rate equal and opposite to the rate of ice movement. In order to accomplish this translation of the ice well, it is necessary to move the walls of ice well 13 which lie adjacent to barge front end 69 and port side 70. It is not necessary to move the walls of well 11 lying adjacent to the barge aft end or starboard side.

Because more energy is required to melt or erode ice than to maintain melted ice in a melted state, it is apparent that it is necessary that more energy be directed into the ice sheet from the barge front and port sides than from the barge aft and starboard sides. It is for this reason that each of the barge side, end and bottom surfaces constitutes at least one discrete energy transfer area. Also, it is for this reason that the use of an openloop system of the type illustrated in FIG. 6, for example, is preferred in cooperation with the barge end and side surfaces. The open-loop heating system, which includes controllable banks of water discharge nozzles 65, allows precise control over the heat flux applied from the barge via each discrete energy transfer area to the walls of ice well 11 at any point around the periphery of the barge.

Assume also that the air temperature above ice sheet 12 is approximately 30F. and that the ice sheet is, say, 8 ft. thick. The temperature of the water below the ice sheet is approximately 28F. Therefore, a temperature gradient of approximately 58 exists across the ice sheet. It is apparent, therefore, that more heat is required to heat a pound of ice adjacent the upper sur face of the ice to the melting point of ice and to melt the ice than is required for a pound of ice located adjacent the bottom of well 11, for example. For this reason, it is desired, as shown in FIG. 7, that water discharge nozzles 65 of open-loop heating system 60 be arranged in horizontal rows spaced vertically along the side walls of barge 17 at different elevations above the barge bottom. Each one of nozzles 65, shown in FIG. 6, represents a plurality of nozzles associated with a particular discrete heat transfer area of the barge. In FIG. 7, however, the nozzles at different elevations along, say, one vertical plane associated with barge w front end 69 are illustrated. Throttle valve 64 is provided to regulate the quantity (volume rate) of hot water supplied from manifold 63 to the front end water discharge nozzles relative to the quantity of water supplied, for example, to barge starboard side 70. Each horizontal row of water discharge nozzles associated with barge front end 69 is connected to valve 64 through a separate control valve 75; if desired, each individual discharge nozzle may have its own control valve associated with it. In general, however, it is sufficient to provide a flow control valve for the nozzles in any given horizontal row along any of the front, side, or aft surfaces of the barge, and it is not necessary to provide a control valve on each nozzle in any given horizontal row of nozzles. Thus, the open-loop ice heating system provided in barge 17 includes valving adequate to enable precise control over the several water discharge nozzles installed in the end and side surfaces of the barge. This control over the amount of hot water supplied to the discharge nozzles on any given side or end wall surface, and the finer control over the amount of water supplied to the nozzles on each surface, make it possible for the waste heat derived from the exhaust of the prime movers aboard barge 17 to be used in the most efficient way possible to enable the barge to accommodate ice movement from any direction relative to the barge.

Therefore, referring to FIG. 9, it is apparent that maximum quantities of hot water would be discharged per minute through the discharge nozzles associated with barge front end surface 69 and port side surface 70, whereas much smaller quantities of water would be discharged per minute through the nozzles associated with the barge aft and starboard surfaces where it is only desired to maintain melted ice in its liquid state.

' Also, it is apparent that regardless of the direction in which ice moves, relatively small quantities of heat need be applied through the bottom of the barge, and in this area closed-loop system 45 is most effective.

It is apparent that control over the heat flux transferred through any heat transfer area could be achieved by controlling the temperature of the heat exchange medium supplied at constant flowrate to any heat transfer area. It is believed that the variable flowrate arrangements described above are more practical in most instances. Since heat is but one form in which energy may be transferred to the ice sheet (see FIGS. 20 and 21), it is seen that this invention comprehends control over the rate at which energy is transferred from the barge to the ice sheet via the several energy transfer areas defined for the barge surfaces. This control provides for most efficient use of energy to maintain the platform at a desired position in the ice sheet relative to a specified submerged location.

It was mentioned above that it is desired that the weight of drilling platform be supported entirely by buoyant means rather than by ice sheet 12. Therefore, during movement of the barge across the ice sheet to compensate for movement of the ice sheet, it is necessary that communication between well 11 and the ocean below the ice sheet be maintained. This necessarily means that hole 37 through the ice sheet from well 11 to the ocean. must also be moved as well 11 is, in effect, moved across the ice sheet. Maintenance of hole 37, regardless of movement of the ice sheet, and movement of the hole during movement of well 11 is assured by providing a heating coil 77 around riser pipe 43 through the ice sheet. The heating coil is stowed in barge center well 26 during air cushion movement of the platform, as shown in FIG. 2. During drilling operations carried out from the platform, the coil is lowered within the center well so as to surround the riser pipe within hole 37, as shown in FIG. 8. The heating coil may be of the electrical resistance type or of the circulating fluid type in which the coil serves a function much like that of coils 50 of closed-loop heating system During the process of installing blow-out preventer stack 40, for example, it may be desirable to totally remove heating coil 77 from within hole 37 so that the blow-out preventer stack may be moved downward through the ice hole. This is not a disadvantageous procedure since it is not a particularly time consuming process to lower the blow-out preventer through hole 37.

Air Cushion Mode Operation Because the structural properties of ice are essentially unknown, it is desired that the weight of drilling platform 10 not be supported by wheels or the like on ice sheet 12 during movement of the drilling platform from one drilling site to another. Reliance upon the structural properties of the ice would mean that the platform could not be moved from place to place during periods in which the thickness of the ice is small. Rather, operation of the platform, particularly movement of the platform from site to site, would have to be scheduled either for periods in which no ice is present or for periods in which the ice sheet is sufficiently thick to carry the weight of the platform. For these reasons, drilling platform 10 is movable in an air cushion mode across the ice sheet from place to place. Since the platform functions primarily as a buoyant barge during a large portion of its useful life, it is desired that .the equipment utilized to enable air cushion mode transportation of the barge from place to place be removable from the barge. Accordingly, as shown best in FIGS. 10 and 11, a platform contemplated by this invention includes a removable air cushion circumferential skirt 84. Skirt 84 is detachably mounted to barge 17 in cooperation with a circumferential air supply plenum 85 which preferably extends around the barge adjacent to the gunwale. The interior of plenum 85 is connected at appropriate locations by suitable ducts 86 to the several air blowers 21 provided within deck house 22.

It is preferred that the structure of plenum be of generally inverted L configuration in cross-section. The horizontal leg of the L defines the top surface 87 of the plenum substantially coplanar with the deck of barge 17. The vertical leg of the L defines'the outer wall 88 of the plenum substantially parallel to but outboard of the upper vertical portion of the barge side and end surfaces. The bottom of the plenum-is open along the entire extent of the plenum circumferentially of the barge except for the provision of rigidifying braces 89 (see also FIG. 3) positioned at appropriate intervals around the length of the plenum.

Skirt 84 preferably is constructed essentially entirely of airtight flexible sheet material such as a rubberized fabric, or the like. In its simplest form, the skirt has inner and outer sheets 90 and 91, respectively, which extend from upper margins 92 adjacent the plenum to lower margins 93 disposed, during air cushion mode operation of the platform, below the bottom of barge 17 be a desired distance. Suitable web sheets 94 are secured transversely between the skirt inner and outer sheets to maintain the desired spacing between these inner and outer sheets during periods when air is supplied to the interior of the skirt under pressure from plenum 85. Web sheets 94 also assist in defining the normal inflated configuration of the skirt.

The specific design and structural aspects of air cushion skirt 84 do not form a portion of this invention. For purposes of example, however, the basic principles and structural features of an air cushion skirt 84 suitable for use with a platform according to this invention are illustrated in FIG. 10. The inner and outer sheets 90 and 91 of the skirt, when the skirt is inflated, are disposed generally vertically relative to the top of ice sheet 12. The inner sheet of skirt 84 is perforated, as at 95, adjacent its lower margin to admit the high pressure air within the skirt to the space provided below barge l7 and the top of ice sheet 12. Blowers 21, therefore, are effective to produce a region of super-atmospheric air between the bottom of the barge and the ice sheet sufficient to support the weight of the barge with the desired clearance above the ice sheet. In one air cushion platform according to this invention, the clearance provided between the bottom of the bargeand the top of the ice sheet during air cushion mode operation is approximately 6 ft., such clearance being produced by air having a plenum pressure of approximately lbs. per square foot gauge pressure. If desired, however, the inner and outer sheets of skirt 84 may be inclined to a vertical reference plane, as illustrated in FIG. 4, for example, and the communication from the interior of the skirt to the region below the barge may be through a space provided between the lower margins of the inner and outer sheets of the skirt.

FIG. 2 illustrates that a plenum 96 for an air cushion skirt may be provided circumferentially of the barge above the barge gunwale and supported relative to the deck of the barge by suitable brackets 97.

Removable attachment of air cushion skirt inner and outer sheets 90 and 91 to the structure of the barge, either directly to the hull or to plenum 85, is facilitated by providing a plurality of apertures 98 through the skirt sheets in their upper margins 92. These apertures preferably are spaced regularly around the circumference of the sheet at a uniform distance below the ex- 

1. A method for maintaining a floating structure in a substantially fixed position in a pool within ice subject to lateral motion irrespective of ice motion, the structure being free of any significant mooring connection to the bottom of a body of water over which the ice is formed, the method comprising the step of: effectively moving the pool laterally in the ice in a direction opposite to the direction of ice motion at a rate substantially equal to the rate of ice motion by applying sufficient energy from within the floating structure to the ice defining a lateral boundary of the pool on a side thereof opposite the direction of ice motion to Reduce said boundary at said rate.
 2. The method according to claim 1 including applying said energy to the ice through the submerged portions of the floating structure.
 3. The method according to claim 1 including moving the floating structure relative to the ice in said opposite direction in conjunction with the pool movement process.
 4. The method according to claim 1 wherein said energy comprises mechanical energy applied to erode said ice.
 5. The method according to claim 1 wherein said energy comprises chemical energy applied to lower the melting point of said ice.
 6. The method according to claim 1 wherein said energy comprises heat applied to melt said ice.
 7. The method according to claim 6 including the step of controlling the rate at which heat is applied to those lateral boundaries of the pool which move toward the floating structure in proportion to the rate at which said boundaries move normal to the floating structure.
 8. The method according to claim 6 including heating at least a portion of the submerged surfaces of the floating structure.
 9. The method according to claim 6 wherein heat is applied from the floating structure to the ice by directing streams of warm water outwardly from at least some of the submerged surfaces of the floating structure.
 10. A method of performing operations at and in association with a submerged marine location overlaid by a sheet of ice susceptible of movement relative to said location comprising the steps of: a. providing a buoyant platform from which desired operations may be performed, b. buoyantly floating the platform in a pool of water communicating through the ice sheet over the submerged location free of any significant mooring connection of the platform to the bottom of the body of water over which the ice is formed, c. performing desired operations at and in association with the submerged location from the platform while the same is in a buoyantly floating state, and d. maintaining the position of the pool over the submerged location, during movement of the ice sheet relative to the submerged location, within limits appropriate to the operations performed at and in association with the submerged location by applying energy from within the platform through at least one submerged location thereof to those boundary surfaces of the pool between the platform and the direction from which the ice sheet moves relative to the submerged location thereby to remove said boundary surfaces and effectively move the pool within the ice sheet.
 11. The method according to claim 10 wherein the desired operations include establishing a well at the submerged location, and including maintaining the position of the pool over the well within a horizontal distance equal to about 10 percent of the water depth between the water surface in the pool and submerged location.
 12. The method according to claim 10 wherein an oil well exists at the submerged location, and the desired operations performed from the platform include the step of production of oil from the well.
 13. The method according to claim 10 including the step of moving the platform into position on a cushion of air to above the submerged location.
 14. The method according to claim 10 wherein the pool of water is defined in the upper extent of the ice sheet, and maintaining communication between the pool and the water below the ice sheet through the ice sheet from at least a portion of the bottom of the pool.
 15. The method according to claim 14 wherein said communication is maintained through a hole in the ice sheet below the platform.
 16. The method according to claim 15 including performing the desired operations at least partially through said hole.
 17. The method according to claim 10 wherein the step of maintaining the position of the pool includes moving the platform relative to the ice sheet in a direction opposite to and at a rate equal to the direction and rate of movement of the ice sheet relAtive to the submerged location.
 18. The method according to claim 17 wherein the platform is moved relative to the ice sheet by connecting mooring lines between the platform and corresponding anchor points in the ice sheet spaced from the platform in substantially orthogonal horizontal directions about the platform, and selectively taking in and paying out said mooring lines from the platform.
 19. The method according to claim 10 wherein the energy-applying process includes applying heat from internally of the platform to the ice sheet.
 20. The method according to claim 19 including controlling the rate at which heat is applied to those boundary surfaces of the pool which move toward the platform in proportion to the rate at which said boundary surfaces move normal to the platform.
 21. The method according to claim 19 including heating at least a portion of the submerged surfaces of the platform.
 22. The method according to claim 19 wherein heat is applied from the platform to the ice sheet by directing streams of warm water outwardly from at least some of the submerged surfaces of the platform.
 23. The method according to claim 22 including mixing in said warm water streams a quantity of a substance which functions to reduce the freezing point of water.
 24. The method according to claim 10 wherein the ice removal process includes eroding the boundary surfaces of the pool.
 25. The method according to claim 24 wherein the erosion process is accomplished by directing from the platform to said pool boundary surfaces streams of water.
 26. The method according to claim 10 including thickening the ice sheet circumferentially of the pool for a selected distance laterally from the pool.
 27. The method according to claim 26 wherein the ice thickening step is performed by applying water over the upper surface of the ice sheet in the area to be thickened and allowing the applied water to freeze.
 28. A method of performing operations at and in association with a submerged marine location overlaid by a sheet of ice susceptible of movement relative to said location comprising the steps of: a. providing a buoyant platform from which desired operations may be performed and which has a weight greater than that capable of being supported by the sheet of ice; b. establishing a pool of water communicating through the ice sheet over the submerged location by applying the weight of the platform directly to the ice sheet; c. performing desired operations at and in association with the submerged location from the platform while the same is in a buoyantly floating state; and d. maintaining the position of the pool over the submerged location, during movement of the ice sheet relative to the submerged locations, within limits appropriate to the operations performed and in association with the submerged location.
 29. The method according to claim 28 including the further step of ballasting the platform following positioning of the platform on the ice sheet over the submerged location to increase the effective weight of the platform.
 30. The method according to claim 28 including heating the ice surfaces adjacent the platform by applying heat from internally of the platform to the ice.
 31. A method of performing operations at and in association with a submerged marine location overlaid by a sheet of ice susceptible of movement relative to said location comprising the steps of: a. providing a buoyant platform from which desired operations may be performed, b. establishing in the ice over the submerged location a pool of water communicating through the ice sheet, c. moving the platform into position over the submerged location on a cushion of air, d. buoyantly floating the platform in the pool, e. performing desired operations at and in association with the submerged location from the platform while the same is in a buoyantly floating state, and f. maintaining the position of the pool over the submerged location, during movement of the ice sheet relative to the submerged location, within limits appropriate to the operations performed at and in association with the submerged location.
 32. The method according to claim 31 including the step of providing the pool in the upper extent of the ice sheet before moving the platform into position over the submerged location.
 33. The method according to claim 32 wherein the pool is formed by explosively fragmenting a volume of ice in the upper extent of the ice sheet corresponding in shape and extent to the pool, and forming a hole through the remainder of the ice sheet thereby to form the pool.
 34. The method according to claim 33 wherein the hole is formed essentially simultaneously with the fragmenting of said volume of ice.
 35. The method according to claim 32 wherein the pool is formed by excavating from the upper extent of the ice sheet a volume of ice corresponding to the shape and depth of the pool, and forming a hole through the ice sheet downwardly from the excavation thereby to flood the pool.
 36. The method according to claim 35 including defining the excavation at least to a depth equal to the sum of 12 percent of the ice sheet thickness and the draft of the platform in a buoyant state.
 37. The method according to claim 35 including moving the platform into the excavation prior to forming the hole through the ice sheet downwardly from the excavation.
 38. The method according to claim 37 including providing a ramp in the ice sheet from the upper surface thereof to the bottom of the excavation, and moving the platform down the ramp into the excavation.
 39. The method according to claim 38 including sealing the ramp from the excavation following movement of the platform into the excavation and prior to forming the hole through the ice sheet downwardly from the excavation.
 40. The method according to claim 39 including removing the seal between the ramp and the excavation upon completion of operations from the platform, and moving the platform out of the excavation up the ramp.
 41. The method according to claim 40 wherein the seal between the ramp and the excavation is defined by a wall of ice, and the seal is removed by fragmenting the ice wall with explosives.
 42. The method according to claim 40 including moving the platform up the ramp by winching the platform relative to the ice sheet.
 43. The method according to claim 42 including melting the ice surface under the platform during movement of the platform out of the excavation and up the ramp.
 44. A method of performing operations at and in association with a submerged marine location overlaid by a sheet of ice comprising the steps of: a. providing a buoyant platform from which desired operations may be performed, b. buoyantly floating the platform in a pool of water communicating through the ice sheet over the submerged location, c. performing desired operations at and in association with the submerged location from the platform while the same is in a buoyantly floating state, and d. thickening the ice sheet circumferentially of the pool for a selected distance laterally from the pool by an amount adequate to compensate for the stress concentrating effect of the presence of the pool in the ice sheet.
 45. The method according to claim 44 wherein the ice sheet is thickened by applying the water to the upper surface of the ice sheet over the area to be thickened, and allowing the applied water to freeze to the ice sheet.
 46. A buoyant structure for use in waters covered by a layer of ice comprising a buoyant hull floatable in a pool formed in the ice, and means for directing energy from a source carried by the hull outwardly from the submerged surfaces thereof in any desired direction to ice defining an adjacent boundary of the pool in an amount sufficient for removing ice at a rate that permits the hull to remain in a substantially fixed position in a movable layer of ice irrespective of the direcTion of movement of the ice relative to the hull.
 47. Apparatus according to claim 46 wherein the energy-directing means is arranged to apply heat energy to the ice.
 48. Apparatus according to claim 47 including means for transferring heat through the bottom surfaces of the hull in an amount sufficient to maintain the hull buoyant in a pool having a depth less than the thickness of an ice sheet in which the pool is formed, and means for maintaining liquid communication through the ice layer from the pool to the underlying water.
 49. Apparatus according to claim 47 wherein the means for directing energy comprises a gas turbine, means for discharging at least one stream of water through the hull to the ice, and means for mixing at least a portion of the exhaust from the gas turbine with said water stream.
 50. Apparatus according to claim 49 including means for adding to the water stream a quantity of a substance which is effective to lower the freezing point of water.
 51. Apparatus according to claim 49 including means for converting the kinetic energy of the gas turbine exhaust to pressure and for applying such pressure to the water stream.
 52. Apparatus according to claim 47 wherein the energy directing means includes means for heating a heat transfer fluid, duct means for directing heat transfer fluid from the heating means to a plurality of discrete areas associated with the hull exterior surfaces, and means cooperating with each discrete area operable to implement transfer of heat from heat transfer fluid directed thereto to the exterior of the hull.
 53. Apparatus according to claim 52 including means for selecting the rate at which heat flux is directed to each discrete area.
 54. Apparatus according to claim 52 wherein the hull has a substantially flat bottom which comprises one of said discrete areas, and means for circulating heat transfer fluid between said one area and the heating means.
 55. Apparatus according to claim 54 wherein the heat transfer implementation means associated with the hull bottom comprises a tank adapted for circulation of heat transfer fluid therethrough and of which the hull bottom constitutes a boundary.
 56. Apparatus according to claim 54 wherein the heat transfer implementation means associated with the hull bottom comprises heat exchange coil means for circulation of heat transfer fluid therethrough and disposed in intimate heat transfer relation to the inner surface of the hull bottom.
 57. Apparatus according to claim 56 including means for chilling heat transfer fluid to below ambient temperature externally adjacent the hull bottom and for circulating chilled heat transfer fluid through the heat exchange coil means.
 58. Apparatus according to claim 54 wherein the hull has side wall surfaces above said bottom, selected portions of the hull side wall surfaces corresponding to different ones of said discrete areas.
 59. Apparatus according to claim 58 including means for circulating heat transfer fluid from each of the different discrete areas to the heat exchange device.
 60. Apparatus according to claim 59 wherein the heat transfer implementation means associated with each of the different discrete areas includes a separate tank adapted for circulation of heat transfer fluid therethrough and of which the corresponding hull side wall portion constitutes a boundary.
 61. Apparatus according to claim 60 including means for stimulating circulation of heat transfer fluid within each hull side wall tank.
 62. Apparatus according to claim 59 wherein the heat transfer implementation means associated with each one of the different discrete areas comprises heat exchange coil means adapted for circulation of heat transfer fluid therethrough and disposed in intimate heat transfer relation to the inner surfaces of the corresponding selected portion of the hull side walls.
 63. Apparatus according to claim 58 wherein the heat transfer implementation means associated with at least one of said differenT discrete areas includes nozzle means for discharging heat transfer fluid supplied thereto to the exterior of the hull.
 64. Apparatus according to claim 63 wherein the nozzle means comprises a plurality of nozzles mounted to the hull side wall at different elevations above the hull bottom for discharging heat transfer fluid supplied thereto to the exterior of the hull, and means for regulating the heat flux transfer from the hull at the different elevations.
 65. Apparatus according to claim 63 wherein the heat transfer fluid is water, and the heat transfer means includes means for supplying water from externally of the platform assembly to the heat exchange device.
 66. Apparatus according to claim 52 wherein the heating means includes means for using at least a portion of the exhaust of an internal combustion prime mover as a heat transfer fluid.
 67. Apparatus according to claim 66 wherein the heat transfer fluid includes water, and the means for mixing said portion of exhaust with water for heating the water.
 68. Apparatus according to claim 67 wherein the heat transfer implementation means associated with at least one of said discrete areas includes nozzle means for discharging heat transfer fluid supplied thereto to the exterior of the hull, and wherein the means for mixing said exhaust and water includes the nozzle means.
 69. Apparatus according to claim 68 including pump means for supplying water from externally of the hull to the nozzle means at a pressure greater than ambient pressure.
 70. Apparatus according to claim 69 wherein the prime mover comprises a gas turbine having an exhaust at high temperature and velocity at substantially atmospheric pressure when operated, and means for converting a portion of the turbine exhaust kinetic energy to pressure energy wherein the exhaust pressure is substantially equal to the pressure of water supplied to the nozzle means.
 71. Apparatus according to claim 70 including means for supplying air at substantially ambient temperature to the nozzle means at a pressure substantially equal to the pressure of water supplied to the nozzle means.
 72. Apparatus according to claim 71 wherein the air supplying means includes a duct connected to the gas turbine in association with the compressor thereof.
 73. Apparatus according to claim 69 wherein the prime mover comprises a gas turbine, and including a blower coupled to supply both turbine exhaust and air at ambient temperature to the nozzle means.
 74. Apparatus according to claim 73 including air skirt means for connection circumferentially to the hull, and means for coupling the discharge of the blower to the air skirt means for supporting the platform assembly on a cushion of air.
 75. Apparatus according to claim 74 including means operable for varying the coupling of the blower from the nozzle means to the air skirt means, and means operable for regulating the temperature of the blower discharge from a temperature above the freezing point of water when the blower and nozzle means are coupled to a temperature below but approaching the freezing point of water when the blower and air skirt means are coupled.
 76. Apparatus according to claim 49 including an operations facility carried by the hull for producing oil from a completed oil well, the facility including means for separating combustible gas from oil derived from the well, and the energy directing means includes means for releasing at least a portion of the latent heat of combustion of said gas and for applying at least a portion of the released heat to said hull surroundings.
 77. Apparatus according to claim 76 wherein the latent heat releasing and applying means includes a gas-fired water heater and means for controllably discharging heated water to externally of the hull.
 78. Apparatus according to claim 77 wherein the water discharging means includes a plurality of water discharge nozzles mounted in the hull for discharging water through the hull.
 79. Apparatus accorDing to claim 78 wherein different portions of the hull exterior surfaces correspond to discrete heat transfer areas, at least one water discharge nozzle is associated with each discrete area, and means for regulating the quantity of water supplied from the heater to the several discrete areas.
 80. Apparatus according to claim 79 wherein some of said discrete areas have associated therewith a plurality of water discharge nozzles disposed at different elevations in the hull, and means for regulating the volume rate of water discharge from the nozzles at different elevations within each such discrete area.
 81. Apparatus for performing operations at and in association with an oil or gas well in arctic areas comprising: a. a platform assembly including a buoyant hull having a substantially flat bottom and configured and arranged to float with shallow draft, b. an operations facility carried by the platform assembly operable to perform desired operations at and in association with an oil or gas well, c. heat transfer means carried by the platform assembly in cooperation with the hull exterior surfaces operable for transferring heat energy through selected portions of the hull which are submerged when the hull is afloat, and d. means operable for supporting the platform assembly on a cushion of air above a water, solid or semi-solid surface.
 82. Apparatus according to claim 81 wherein the air cushion support means includes an air cushion skirt assembly, and means for removably connecting the skirt assembly to the platform assembly circumferentially of the hull.
 83. Apparatus according to claim 82 including means in the platform assembly operable for supplying air at greater than atmospheric pressure to within the skirt assembly below the hull, and the heat transfer means includes means for adjusting the temperature of air supplied to within the skirt assembly to a temperature between ambient temperature and the freezing point of water.
 84. Apparatus according to claim 83 wherein ambient temperature is substantially below the freezing point of water, and the air temperature adjusting means is operable to adjust the temperature of supplied air to a temperature approaching but below the freezing point of water.
 85. Apparatus according to claim 84 including a blower, a prime mover having a high temperature gas exhaust when operated, and means for supplying a controllable quantity of prime mover exhaust to the blower.
 86. Apparatus for performing operations at and in association with a submerged oil or gas well in arctic areas comprising: a. a platform assembly including a buoyant hull defining therein a well open to the bottom of the hull, b. an annular heat transfer device adapted for storage in the well; c. means for disposing the heat transfer device in an operative position at least partially below the hull adjacent the well; and d. an operations facility carried by the platform assembly operable to perform desired operations at and in association with an oil or gas well through the annular heat transfer device during periods when the hull floats in a water pool formed in the upper extent of an ice sheet and is the heat transfer device operative to maintain water communication between the water pool and water over which the ice sheet is formed.
 87. A method for maintaining a floating structure in a substantially fixed position in a pool within ice subject to lateral motion irrespective of ice motion, the method comprising the step of: effectively moving the pool laterally in the ice in a direction opposite to the direction of ice motion and at a rate substantially equal to the rate of ice motion by applying thermal energy outwardly from the submerged surfaces of the floating structure to the ice defining a lateral boundary of the pool on a side thereof opposite the direction of ice motion. 